Who played the biggest role?

British, US and Canadian troops bore the brunt of the fighting on 6 June 1944.

Some 156,000 men landed on the five beaches earmarked for invasion – Utah, Omaha, Gold, Juno and Sword – or landed nearby in gliders or parachutes.

Of these, 73,000 61,715 were British, 73,000 were American and 21,400 were Canadian.

The historian James Holland estimates that the invasion featured 200 US and 892 British warships. Of the more than 4,000 landing craft, 3,261 were British and 805 American.

Smaller numbers of soldiers from other countries were also involved, including Australia, Belgium, Czechoslovakia, France, Greece, the Netherlands, New Zealand, Norway and Poland.

Estimates of casualties are probably still inaccurate, but the US National D-Day Memorial Foundation puts the death toll at 4,413 – 2,499 Americans and 1,914 from the other Allied countries combined.

The supreme commander of Allied forces was the US general Dwight D Eisenhower. Britain’s General Bernard Montgomery drew up the plans for the invasion and was in charge of ground forces.

Non-German troops played a big part in manning Rommel’s “Atlantic wall” defences too: many men in German uniform were conscripts from occupied countries like Poland and Ukraine.

The presence of large numbers of foreign soldiers on the German side has led to the perception that the Allies had an easier time than their Russian counterparts.

But historians like Antony Beevor have stressed that many elite German units, including SS Panzer divisions, seasoned veterans of the eastern front and crack parachute troops were thrown into the fight.

Was it the biggest military operation in history?

By 11 June about 326,000 troops, 54,000 vehicles and 100,000 tons of supplies had been landed on the beaches.

The actual scale of the fighting on D-Day and in the weeks that followed were dwarfed by the colossal battles of the Eastern Front, where Axis and Red Army forces deployed millions of men.

How close did it come to failure?

Eisenhower famously wrote a letter taking full responsibility in case the landings failed, and carried it in his pocket until he was certain that a beach-head had been established.

Military historians have attributed the Allies’ succes to a combination of successful subterfuge and good fortune.

The Germans were expecting an invasion, but a long campaign of misinformation led them to believe the blow would fall on Calais, where the port would make it easier to unload men and equipment.

MI5 double agent Juan Pujol, codename Garbo, played a crucial role in convincing German commanders the attack on Normandy was just a feint designed to divert their attention from the real target of Calais.

Garbo’s lies were bolstered by the creation of a whole fake army in Kent and Essex, complete with inflatable tanks and trucks designed to fool spy planes.

Hitler dithered and the order to send armoured divisions to launch a counter-attack came too late.

With such a complex operation, many things could have gone wrong.

One paratroop officer, Lieutenant Colonel Terence Otway, decided to test the resolve of his men by deploying pretty female auxiliaries from the RAF to try to charm information out of them in local pubs. But no one breathed a word about the invasion plans.

The weather proved to be a decisive factor. Navy meteorologists correctly predicted that the wind would drop and the skies would clear for a while on 6 June after a spell of bad weather.

Were the plans leaked?

In May 1944 the Telegraph printed a crossword including the word Utah as one of the answers.

The codenames of other beaches – Juno, Gold and Sword, had all appeared in previous puzzles.

On 22 May the name of the fifth D-Day beach, Omaha, appeared. Later that month, Overlord, the codename for the whole invasion, Mulberry (the name of the artificial harbours to be used in the landing) and Neptune (the amphibious assault) were all solutions.

MI5 had already been keeping an eye on the Telegraph crossword since 1942, when the word Dieppe cropped up the day before the failed raid on the Channel port.

Crossword setter Leonard Dawe was interrogated by MI5 but the spooks decided the whole thing was probably a coincidence.

In the 1980s, pupils at the school where Dawes was headmaster claimed that they had helped compile the crosswords and had inserted the names of codewords they had overheard from soldiers camped near the school.

Did Churchill and Roosevelt drag their feet?

Soviet dictator Joseph Stalin had been calling for Britain and America to open a second front against Hitler since 1942, to relieve the intense pressure on Red Army forces in the east.

Historians disagree on whether Britain’s prime minister, Winston Churchill, was simply being prudent to wait until the Allies could amass overwhelming numbers of men before attempting an invasion.

Russian writers have suggested that Churchill and US President Franklin Roosevelt deliberately stalled their invasion plans because they wanted to weaken Russia.

Sir Max Hastings wrote: “In his less guarded moments Churchill was by no means unhappy to see the Russians doing the bleeding and the dying that otherwise the British would have had to do.”

Would the Russians have won the war anyway?

Historians are also split on the question of whether Stalin could have beaten Hitler single-handedly, if the Normandy invasions had been called off or repulsed.

Key victories at Stalingrad and the Kursk had seriously weakened Germany. In fact, historians estimate that the Red Army inflicted between 75 per cent to more than 90 per cent of all German casualties in the whole of World War Two.

General Sir Richard Dannatt, former head of the armed forces, wrote: “The Russians would probably have continued their advance towards Western Europe, but at a slower pace due to more German reserves being available to be deployed against them.

“The intriguing question is: could the Allies still have managed to get an unconditional German surrender, or would there have been a negotiated end to the war? In that case, what would Europe look like today?”